1. Field of the Invention
This invention relates to a heat-sensitive recording sheet. More particularly, the present invention relates to a heat-sensitive recording sheet comprising (1) a substrate, (2) a heat-sensitive, color-developing layer provided on said substrate, composed essentially of a colorless or light-colored dye precursor and a color developer capable of allowing said dye precursor to develop a color by reacting with the dye precursor when heated and, as necessary, (3) an overcoat layer provided on said heat-sensitive, color-developing layer.
2. Description of the Prior Art
Thermal recording has many advantages such as it requires no impact and makes no noise during recording, neither development nor fixing is required and equipment maintenance is easy. Accordingly, it is used in many applications such as bar code labels, tickets and the like through means such as printers, facsimiles and the like.
Heat-sensitive papers used in thermal recording are firstly required to have a color development sensitivity which is appropriate for intended applications. In addition, they have various other requirements such as (1) their background is white, (2) the developed image is stable and does not disappear over a long period of time, (3) during recording, the heat-meltable components do not stick to a thermal head of a printer as work-ups and accordingly printing is not impaired and (4) also during printing, the heat-meltable components do not fuse to a thermal head and accordingly no difficulty of paper feeding arises and no noise is developed and no hindrance of image development (no sticking) occurs. Further, in coating of a heat-sensitive color-developing layer to produce a heat-sensitive paper, there should be no repellency, no pin hole and no unevenness. That is, the heat-sensitive color-developing layer should have an excellent levelling property because it can greatly contribute to printability. Furthermore, heat-sensitive papers should cause no blocking between the face of a paper and the back side of another paper when left in an "as wound-up" state after drying. When heat-sensitive papers are subjected to a finishing step by a super calender, the heat-sensitive layer should not cause partial sticking (dusting).
In heat-sensitive papers, for prevention of work-up formation, a pigment having a high oil absorption is generally used in the heat-sensitive color-developing layer, or, for prevention of sticking, a metallic soap is generally used in said layer. However, the use of such substances not directly participating in a color development reaction results in a dilution effect whereby the developed image density and/or the color development sensitivity of these heat-sensitive papers are/is reduced. Further, for prevention of repellency, pin holes and unevenness appearing at the time of coating a heat-sensitive color-developing layer to produce a heat-sensitive paper, various surfactants are being trially used in said layer; however, the use is not satisfactory because it causes (a) excessive infiltration into paper, (b) foaming and (c) deterioration in water resistance of the layer. Furthermore, for prevention of blocking and/or dusting, it is practised that the adhesive used for formation of heat-sensitive color-developing layer is made water-resistant; however, this operation can not be completed in a short time requiring 1 to 2 weeks because high temperatures can not be used in drying.
Also, in handling of heat-sensitive papers after recording, difficulty of reading of developed images due to disappearance of developed image or background fogging occurs when the papers come in contact with a rubber eraser or a polyvinyl chloride bag, both containing a large amount of a plasticizer, or when the papers are touched by hands stained with a hand cream or a hair oil, or when such a paper and a diazo type photo-sensitive paper are superimposed on each other right after development, or when a filling-in such as underlining is made with a fluorescent pen. This image disappearance phenomenon is believed to take place because the lactone ring of a color-forming lactone compound which has been opened by an acidic color-developing agent is closed in the presence of a plasticizer such as dibutyl phthalate, dioctyl phthalate, dioctyl adipate or the like. The fogging phenomenon is believed to take place because a solvent contained in a developing solution for diazo type photo-sensitive paper or in a fluorescent pen dissolves a color-developing agent and as a result the color-developing agent comes in contact with the dye precursor to develop a color.
It was disclosed in Japanese Patent Application Kokai (Laid-open) Nos. 128347/1979 and 3549/1979 that the above phenomena can be prevented by forming an overcoat layer on the heat-sensitive color-developing layer of a heat-sensitive recording paper because the formation of an overcoat layer prevents infiltration of a plasticizer into said paper.
An overcoat layer having chemical resistance was proposed by the present inventors in Japanese Patent Application Nos. 182557/1980 and 981/1981 (Japanese Patent Application Kokai (Laid-open) Nos. 105390/1982 and 115391/1982, respectively).
In formation of an overcoat layer according to the above conventional arts, however, there tended to exist repellency, pin holes, unevenness, etc. and thick coating was necessary to obtain satisfactory chemical resistance. This not only required a long time for drying, slowed the coating speed, which became problems in production, but also caused the decrease in the sensitivity of color development. Recently, a faster printing speed is being adopted and therefore it is necessary that any coating layer be made as thin as possible for smaller heat loss. Hence, improvements in repellency, pin holes, unevenness, etc. are a task to be urgently solved in view of requirements for coating layer.
In order to improve the levelling property of an overcoat layer, an attempt was made, as in the case of a heat-sensitive color-developing layer, to use various surfactants in an overcoat layer. However, the use greatly reduced the water resistance and oil resistance of the overcoat layer as well as the retainability of the quality of the background and recorded portions. The levelling property can be improved to some extent by the use of a pigment; however, the pigment can not be used in a large amount because the use induces hindrance of film formation, reduction of color development sensitivity and wear of a facsimile thermal head. Thus at the present stage, these approaches for improving the leveling property of an overcoat layer are not satisfactory.